1. Field of the Disclosure
The disclosure generally relates to analogs of tetrahydrobiopterin, compositions containing the same, and methods of treating an individual suffering from a condition responsive to tetrahydrobiopterin by administration of the analog.
2. Brief Description of Related Technology
Tetrahydrobiopterin (also referred to herein as “BH4”) is a naturally-occurring chemical compound and is a biologically active amine of the pterin family. One stereoisomer, sapropterin, is shown in Formula II, below:

Although naturally-occurring, tetrahydrobiopterin also may be synthesized by a variety of methods, some of which are disclosed in, for example, U.S. Pat. Nos. 2,601,215; 3,505,329; 4,540,783; 4,550,109; 4,587,340; 4,595,752; 4,649,197; 4,665,182; 4,701,455; 4,713,454; 4,937,342; 5,037,981; 5,198,547; 5,350,851; 5,401,844; 5,698,408; and, 5,698,408, and Canadian patent application No. 2,420,374.
Pterins are bicyclic compounds that include a pyrazine ring and a pyrimidine ring having a carbonyl oxygen and an amino group. Pterins function as cofactors in enzymatic catalysis. Tetrahydrobiopterin functions as a cofactor for a number of different enzymes, including phenylalanine hydroxylase (PAH), tyrosine 3-hydroxylase, tryptophan 5-hydroxylase, and all three forms of nitric oxide synthase (NOS). Tetrahydrobiopterin also is a growth factor for Crithidia fasciculata, has proliferative activity in haemopoietic cells, and acts as a self-protecting factor for nitric oxide toxicity. These and other cofactor and cellular functions of tetrahydrobiopterin as well as disorders relating to tetrahydrobiopterin deficiency are disclosed in Thony et al. (2000) Biochem. J. 347:1-16. Disorders relating to tetrahydrobiopterin deficiency also are generally described in Blau et al., Disorders of Tetrahydrobiopterin and Related Biogenic Amines, in The Metabolic and Molecular Bases of Inherited Disease, 1275-776 (8th ed., McGraw-Hill Publishing Co., New York, N.Y., 2001).
Tetrahydrobiopterin is a hydrophilic compound that has difficulty crossing membranes as well as traversing the blood-brain barrier. The blood-brain barrier generally is a membrane that controls the passage of substances from the blood into the central nervous system (CNS). It functions as a physical barrier between local blood vessels and most parts of the CNS, preventing certain (and many) compounds from reaching the CNS. The walls defining capillaries in the body are made up of endothelial cells separated by small gaps. These gaps permit soluble chemicals within tissues to pass into the blood stream, so that the chemicals can be carried throughout the body, and subsequently pass out of the blood into different tissues. In the brain, these endothelial cells are packed more tightly and, therefore, the gaps are even smaller. These smaller gaps block the passage of all molecules except those that cross cell membranes due to lipid solubility (e.g., oxygen, carbon dioxide, ethanol) and those that pass by specific transport systems (e.g., sugars, select amino acids). Many drugs do not cross the blood-brain barrier in amounts effective to provide therapy. In addition to providing a physical barrier to the CNS, endothelial cells in the brain also may metabolize certain molecules (drugs) so that they never reach the CNS.
The present invention is directed to more effective ways of delivering tetrahydrobiopterin to the body as well as to the CNS to provide effective therapy for disorders and conditions responsive to tetrahydrobiopterin.